Television system



Oct. 12, 1937. E. l. GREEN .A TELEVISION SYSTEM Filed April 28, 19:52

4 Shee'ts-Sheet 1 ATTORNEY 4 sheets-slm@ 2 l E. GRI-:EN

TELEVISION SYSTEM Filed April 2s, 1932.'

oct. 12, 1937.

4 Sheets-Sheet 5 E, l, GREEN TELEVISION SYSTEM Filed April A28, 1932 Oct. 12, 1937.

/Nl/E/vro@ E GREEN 5:/

Ar rom/Ev Oct. 12, 1937.

E. l. GREEN TELEVISION SYSTEM f Filed April 28; 19:52

4 Shee'ts-Sheer. 4

ATTORNEY l 1 Patented Oct.*12,l 1937 PATENT OFFICE j TELEVISION srs'rEM Estill I. Green, East Orange, N. J., assignor to American Telephone and, Telegraph Company,

a corporation of New York Y,

l Application' April 28,. 193,2, .serial No. 608,113 26 claims. (ci. 17a- 5.6)

This invention relates to television systems and more particularly to methods and apparatus whereby an image band, which extends from substantially zero frequency to a frequency of conslderable value, may be eilciently transmitted andfaithfully reproduced at a receiving station.

In one aspect the invention is concerned broadly with systems. for distributing to a number of subscribers one or more television programs, which may, if desired, be accompanied by sound programs, The arrangements described may be applied to a distributing network of the type commonly used for telephone purposes.

The linvention is especially adapted for use in broadcasting systemsfwherein a combined television and associated sound program may be transmitted over a single communicating channel. v

A feature of the invention Iis the provision of apparatus at a central-point for shifting the position of the television band, which point serves a large distribution area, whereby duplication of this apparatus is'avoided.

The prior art includes a television system in which `the image current band is separated by filters into two portions, the lower of which modulates a carrier wave of such frequency that the resultant modulation products occupy a position in the frequency spectrum above the highest frequency component included in the upper portion. The-upper portion of the image band is transmitted to a receiving station along with the modulation products which are selected and detected Cil to yield the lower portion of the image current band, which is passed through attenuating and phase correcting networks and the detected components are combined with the directly transmitted portion to faithfully reproduce the image current band. i

There is also disclosed in the'prior art, a system which produces an image band in which the energy is largely concentrated in a number of distinct frequency bands between which there is very little useful energy. This band is split into two sections, which are used to modulate carrier cur.-

rents of respectively different frequencies, two

sidebands andv the carrier resulting from one modulation are transmitted along with one sideband of the other modulation; At the receiving station the incoming image modulated currents based on the respective carrier frequencies are separated by filters and separately detected toI In another system heretofore proposed, the transmitting terminal apparatus produces an image current band and also a carrier' wave modulated by the image band both of which are amplif ed by an alternating current device, which op-r upper limiting frequency of the image band,

whereby the composite current occupies'a frequency range only slightly greater than the width of the image band. This composite current is transmitted to a receiving station, where the distorted mid-portion ofthe modulated wave is selected and stepped down in frequency so thatv one of its components is at zero frequency and its other components are in positions to respectively complement the distorted components of the portion 'of the image band directly transmitted, and the two sets of distorted components are combined whereby the image band is faithfully reproduced.

According to one embodiment' of the present invention, the image current band is divided into upper and lower portions, the lowerportion modulates a carrier current whereby there are produced two sidebands and an unmodulated component of carrier frequency, the upper portion of the image bandfmay be combined with a second carrier having ajfrequency different from the rst mentioned carrier but of such value that the resultant sidebands are spaced apart sufficiently to permit a lter to select one sideband while substantially suppressing the other, the selected sideband is used to modulate a third carrier which results in the production of two side bands spaced apart sufficiently to permit their being separated by a filter and having modulated components based'on the same frequencyas the unmodulated `component of the modulated current including the lower portion of the image band, or the upper portion may be used to modulate a carrier of the same frequency as that modulated by the lower portion ofthe image band, but separately therefrom, to produce two sidebands spaced apart sufficiently to permit a filter toy select one sideband while substantially suppressing the other. In either case one sideband ofthe products including the upper portion of the image band is selected and transmitted along with the modulation products including the lower portion of the image band and the unmodulated carrier component.

In accordance with the above described embodiment of the present invention there is produced for transmission an image modulated carrier current which, while having a much narrower band width than a wave. comprising two sidebands, comprises modulated components and an unmodulated component that are directly combinable by detection to faithfully reproduce the image current.

This vinvention utilizes a method for producing an image-modulated carrier current which permits the use of a simple`type of receiving set or, 'in other words, one which does not require technical training to operate.

A more detailed description oi' the invention follows and is illustrated in the attached drawings, in which, V

.Figures 1 and 2, with the latter at the right of Fig, 1, diagrammatically illustrate a system in 'which a number of combined television-sound programs maybesupplied to one or more central offices each provided with selecting mechanisms adapted to' be controlled from subscribers stations, to permit selection of any one of the programs which a subscriber desires to receive;

Fig. 3 shows a terminal apparatus for the combined television-sound program, which may be substituted for that shown in Fig. 1;

Fig. 4 illustrates another terminal apparatus which may be substituted for that shown in Fig. 1 or in Fig. 3; Y

Fig. 5 illustrates receiving apparatus which maybe substituted for that shown in Fig. 2;

Fig. 6 illustrates an arrangement of transmitting terminal'apparatus which may be substituted for that shown in Fig. '1;

Fig. 7 illustrates yet another arrangement of transmitting terminal apparatus which may be substituted for that shown in Fig. 1; and

Fig. 8 illustrates an embodiment of the invention in which separate subscribers circuits may be employed for the sound signals and the television signals.

As shown in Figs. 1 and 2, the latter being placed to the right of Fig. 1, the system comprises a program center PC at which are produced currents corresponding to a number of programs, each comprising a eld of view to be transrelated sound currents, which may be transmitted over a trunk T (circuits I1 and 26) to a central oilice CO. This oice includes a program selector PS associated with each subscribers loop or line SL and controllable thereover from a subscribers station, whereby a subscriber may connect his line through circuits, such as 3|, to any one of the trunks T to select the program, comprising the respective image and related sound currents, which he may desire to receive.

'I'he terminal apparatus for one television sound channel is shown-in more or less detail, a second is schematically illustrated and a number of others are indicated by lead wires LW.

The television seaming apparatus TS at the program center may include an optical system comprising a source of light I, radiations from which are concentrated into a beam by a lens system 2 and applied to the face of a disc 3, provided with a series of apertures arranged in a spiral line, whereby there is produced a moving beam of light of great intensity which passes through the opening in a mask 4 and a lens l to illuminate successive elemental areas of Vadjacent parallel lines of a field of view li herein shown as the face of a speaker or entertainer. The opening in mask 4 determines the size of the field to be scanned, the aperturesvare of such size thatV disclosed in United States application Serial No.

227,649 of F. Gray, filed October 21, 1927.

The resultant image current comprises current components constituting a frequency band which extends from substantially zero frequency to an upper limiting frequency, the width of which depends largely upon the amount of detail to be transmitted and may-be many kilocycles.

Associated with the television scanning mechanism is a microphone 8 arranged to pick up the speech delivered by a speaker or the vocal sounds or instrumental music produced by lany entertainer or other appropriate sounds. Current corresponding to this portion of the program may extend from 30 cycles to about 10 kilocycles.

Since the image and sound currents constituting a program occupy overlapping portions of the -frequency spectrum and are simultaneously transmitted over a single channel from the oentral oiiice to a subscriber, and the separate pro' grams are transmitted from the program center to the central office over adjacent trunks, between which cross-talk may occur, two problems are involved. The image and sound currents must be transmitted so as to avoid interference of one with the other, and the several programs must be transmitted in such manner that cross-talk between trimks does not cause objectionable interference and distortion. In view of the fact that the requirement as to cross-talk between different sound channels appears to be more severe than in the case of television channels, it is desirable to displace the image current to a position in the frequency spectrum outside the range of the audible current. That is, the sound should be transmitted in the lower frequency range where cross-talk is less liable to cause interference and distortion. Also it may be desirable to avoid using the lower frequencies for television transmission because of the phase difliculties encountered, due to the fact that the phase shift of the transmission line or channel is not uniform for the current components of different frequencies within the image band.

The simplest method for shifting the television signals, to a frequency position above that of the sound signals is to combine them with a carrier current and transmit both sidebands and the unmodulated carrier component. Since this expedient is uneconomical of the frequency spectrum, the system to be herein described embodies an arrangement for producing anl image modulated carrier current having a band width only slightly greater than that involved in single side- 75 may be provided while using -a frequency range rent, will be only cycles or thereabouts. Again,

since modulators operate to repeat the current impressed on their input circuits and the prsence of suchrepeated components may result in distortion, it is desirable to use a carrieicurrent of such frequency that the modulation 'products occupy a position in the frequency spectrum outside the range of the image current ban'd which Y may comprise components of many hilocycles.

'I'his necessitates the use of a carrier current of such high frequency that it is impracticable; with present day technique, to provide a carrier fre-- quency band filter which will select one sideband of the modulated current and suppress the other sideband, when the frequency spacing between Complete suppression the sidebands is so small. of the sideband is desirable, because if any components of the undesired sideband are not suppressed, they will, upon demodulation, coincide with the corresponding frequency components of the useful sideband and, because of their varying phase relation wlthrespect to the desired components of corresponding frequencies, they may produce an irregular transmission characteristic for the lower frequencies of the image band.

Uniform transmission of the various components of the image band, while only slightly increasing the frequency range over that occupied by the band itself. is accomplished by splitting the band into two unequal portions, effecting separate modulations with these portions, and combining, the modulation products including the smaller portion of the image band with one sideband of the modulation products including the larger portion of the image band. This is effected by supplying the image current, produced by the photo-electric tube 'I over branch circuits respectively including ampliers 9 and I0 to a low pass filter II anda high pass band lter I2, respectively included in the output circuits of the'.

amplifiers 9 and I0.

Low pass flter- II is designed to freely transmit, substantially without attenuation, current components extending from zero frequency to an upper limiting'frequency, for example, 500 cycles, and to suppress current components of higher frequencies. On the other hand, band lter I2 freely transmits, substantially'without attenua. tion, current components extending from the cutoff frequency of filter II to and suppresses those within the transmission range of filter II.

\At a frequency as low as 500 cycles, filters II and I2 can be designed to cut off with extreme sharpness, and, hence they will serve to effect the desired separation of the image band into lower and upper portions, respectively including current components from 10 cycles or less up to 500 cycles and 500 cycles to an upper limitingyfrequency. Furthermore, the nature of the television signals occurring in the low frequency range is such that practically all of the useful energy is concentrated in certain parts of ythe .frequency range, the location of these parts being determined by the number of pictures transmitted per second, the number of apertures in the scanning disc, and similar factors. Between these points of energy concentration, there are other points of the frequency range in which there is substantially no energy, a`nd it is intended that separation of the image band is to be effected at -some one of these points where the energy is substantially zero.

'I'he lower portion of the frequency band is supplied `by filter II to a modulator I3 in which it is combined with a carrier Vcurrent whose frequency is assumed to be 15 kc. supplied by a source I4, to produce modulation products comprising two sidebands and an unmodulated carrier component. These products, comprising a side bandl (14.5 kc. to 15 kc.), an unmodulated carrier component of 15 kc. and a sideband (15 kc. to e 15.5 kc.), are transmitted through the filter I5 and, after being'ampliiied by the amplier I6, are supplied to a trunk circuit I1.

l When it is desired toshift the original wide band of television frequencies upwardly inthe frequency range by an amount 4which represents only a fraction of its total width and to use tor of input frequencies which overlap the desired output band,V aswell as difficulties which might result if the sideband to be suppressed included current components lying within the frequency range occupied by the side band to be transmitted,

a's will be'hereinafterv explained.

In.accordance with the scheme employing two stages of modulation, the upper portion of the image band, transmitted through lter I2, is supplied to. a modulator I8 in which it is combined with a `carrier current which is assumed to be 180`kilocycles. This frequency may be supplied by a harmonic producer I9, which in turn is supplied with current of 15 kilocycles from the source I4. The path connecting the harmonic producer with the modulator includes a circuit 20, 4.

tuned'to the 12th harmonic of the current supplied by source I4.,

The energy flowing through the output circuit of modulator I8 comprises two sidebands' and an unmodulated component of carrier frequency (180 kilocycles). The two sidebands arel separated from eachother by a frequency spacing of 1000 cycles and each is separated from the carrier component by a spacing of `500 cycles and by a greater spacing from the repeated current components of the upper portion of the image band.

The two sidebands, associated carrien and other components supplied bymodulator I8, are impressed upon a lter 2l which freely transmits the upper sideband and suppresses the unmodulated component, the lower sideband and the re- I vpeated components of the image band. As a matter of fact, filter 2I may transmit some of thl unmodulated carrier component and certain components of the lower sideband adjacent the carrier, without causing any detrimental effect in system, as will be explained of, for example, 185 kilocycles, supplied by the V lated carrier component of 165 kilocycles and other components, including the single sideband impressed upon theinput circuit of modulator 22 and perhaps certain other components whichwere not suppressed by filter 2|. This energy is supplied to the band-pass filter 24 adapted to select the lower sideband, comprising current components having frequencies extending from 15.5 kilocycles to an upper limiting frequency which for present purposes may be assumed to be kilocycles. Any current components of the undesired sideband repeated by the modulator 22 without modulation, or which combine withA the -kilocycle carrier current to produce modulation products, will be so far removed from the cut-off frequency of filter 24 that they will be suppressed thereby. For example, the carrier component of kilocycles and the sideband components adjacent thereto in frequency will be suiiiciently farA removed from the upper cutoff frequency of .iilter 24 to assure that such undesired components will be suppressed. The 15kilocycle current component, produced by combining the carrier of 180 kc. passed by filter 2l and the carrier of 165 kc. supplied to modulator 22, will be removed from the lower cut-off of lter 24 by 500 cycles, which is ample at this point in the frequency spectrum to assure that it will be suppressed by this filter. components lying below 15 kc. which result from incomplete suppression in filter 2| will be readily suppressed. Y

The 15.5 kc. to upper limiting frequency band transmitted by filter 24 after being amplified by the amplifier IS, is supplied to the trunk I1 over which it is transmitted to the central oilice. Since this operation occurs simultaneously with the modulation by the lower portion of the image band, previously described, trunk I1 will be supplied with a modulated wave made up of two side bands lying between 14.5 kc. and 15.5 kc., a single sideband extending from 15.5 kc. to an upper limiting frequency', determined by the frequency width of the upper portion of the image current, and an unmodulated carrier component of 15 kc., with which the two sidebands, includamplier I 6 to the trunk I1, respectively, may;

be connected to other trunks over circuits indicated by the taps 21 and 28, respectively;` whereby the program initiated at the program center may be transmitted to any number of other central ofiices for distribution to subscribers associated with these oiilces. At the central oilice the incoming audible frequency Vcurrent is amplified by amplier 29 and transmitted through the low-pass nlter 30 to the circuit 3|. At the same time, the composite image modulated current is amplified by the amplifier 32 .and transmitted Likewise the conductors of which are cojnnected to contacts a .of two banks of contacts respectively associated with movable wipers 34 of a stepby-step switching device forming part of the program selector PS.

Contacts b of the contact banks are connected to the conductors of a second circuit 3| over which a second program, represented by an image modulated carrier current and audible frequencyv current corresponding to associatedsound, similar to those of the rst program, may be received from a second transmitting apparatus 35 (Fig. 1), situated at the program center, which is identical with that described above. Other similar transmitting apparatuses at the program center may be connected over similar circuits to the successive contacts c, d, etc., of the bank, as indicated by the leads LW. A

Circuits 3|, 3|', etc., may be connected to other similar step-by-step switching devices each associated with a diierent subscribers line, as indicated by the taps I3l and I3l. The stepby impulses transmitted from a subscribers sta-3 tion, in a mannerto be hereinafter described,

to connect his loop or line circuit SL to any" desired one of the several trunks and'therethrough to a desired program transmitter.

Wipers 34 are connected by contacts 36 and armatures 31 of a cut-off relay 38 to the primary winding of a repeating coil 39 having its secondary winding 40 included in the loop or line circuit SL extending to a subscribers station and terminating thereat in a repeating co'il 4| (Fig. 2) having its secondary winding connected to a branched circuit. One branch 42 of this circuit includes a low-pass lter 43, adapted to freely transmit the audible frequency current and prevent the passage of the image modulated current. The audible frequency current is amplifled by the amplifier 44 and the amplified current is supplied to sound reproducing device 45, herein shown as a loud speaker.

The other branch 46 includes a high-pass filter 41 which prevents transmission of the audible frequency current, but permits the 'image modulated and unmodulated carrier current to pass to a demodulator 48.

While the modulated and unmodulated components of the incoming wave may be directly combined to reproduce the image current, this method of operation may be undesirable. 'I'he reason for this is that the incoming modulated vcurrent may be repeated in the output circuit of the demodulator and since the modulated current and image current occupy overlapping portions of the frequency spectrum, there will be present in the output circuit of the demodulator two groups of current components having identical frequencies, which tend to interfere with each other. `To .overcome this objection and thereby eliminate all possibility of interference, it is preferable tol use a receiving apparatus utilizing double detection.

By employing oscillations of the proper fre-v quency in the rst stage of demodulation, the output current of the respective demoduators may be caused to occupy different frequency ranges from that of the current impressed upon their input circuits.

In accordance with this lmethod of operation the demodulator 48 is supplied by an oscillator' 48 with oscillations which combine with the image modulated current to step it up in frequency toa position in the frequency spectrum such "that the frequency band produced in its output prlsing a discharge lamp |00, supplied with the -image current, associated with a' rotating disc similar to disc 3 at the transmitter, which is driven by a motor |02 to produce an image of the subject scanned for transmission within the period of persistance of vision. The television receiver operates to produce an image which is directly viewed by an observer. Any other suitable receiver for this purpose, or for projecting the image on a screen, may be used ln place of the receiver described above.

To produce an image of the subject scanned at the program center, it is essential that the scanning disc |0| be operated in synchronism and in phase with the scanning `disc 3 of the television pickup TS. Any suitable means may be used for this purpose, as for example, that disclosed in U. S. Patent 1,874,200l of E. C. Manderfeld, issued August 30,` 1932. Brielydescribed, this' system operates as follows: At the transmitter, the opening in the mask 4, in the direction of the scanning lines, may be made less than the distance between 'centersof adjacent apertures inthe scanning disc, so that the scanning light beam is periodically interrupted at the end of each line before the scanning of lthe succeeding line is initiated, or the width of the opening in the mask 4 may be made such that the last element of one line and the first element of the succeeding line are scanned simultaneously, thus periodically increasing the light which activates the photosensitive device 1 to a, high value. In either case, there will'be introduced into the image current a corresponding frequency component of relatively high value, which is selected at the receiving station and utilized to control the speed of motor |02. For a complete disclosure of the operation of this synchronizing system, reference may be made to the above mentioned Manderfeld patent.

The advantage of utilizing successive modulation and demodulation at the program center and at the receivers, respectively, will be evident from a consideration of the -following numerical example. Let it be assumed that the television pickup at the program center operates to produce an image current occupying-the frequency range from approximately 10 cycles to 85 kilocycles and that the program is to be transmitted over a channel having a frequency capacity of 100 kilocycles. As stated above, the audible signal current extends from 30 cycles to about 10 kilocycles and, in order to provide the required frequency spacing to permit this current to be separated from the image modulated carrier current, by filters at the receiver, the image band modulates a carrier current of 15 kilocycles. I

If, as described above, the upper portion of the image band is used to effect modulation in one step, there will be present in the output circuit of the modulator, used to combine this portion (500 cycles to 85 kilocycles) with the carrier curent, a sideband extending from about 15.5 kilocycles to 10|) kilocycles, and a sideband extending from v ing wave.

14.5 kilocycles down to zero frequency and upward to '10kilocycles, and due to the repeating action of the modulator, an image band extending from about cycles to 85 kilocycles, together with an unmodulated component of kilocycles. Although the amplitudes ofthe repeated components may be made small, compared with those of like frequency in the modulation products, by utilizing a carrier current of large amplitude; the components of the unwanted sideband which overlap those of the Wanted sideband may cause interference, resulting, after demodulation, in

distortion of the image.

However, by rst combining the upper portion of the image band with a carrier of 180 kilocycles, the modulation products occupy a. different' frequency range from that of the impressed image band, and the two sidebands are so spaced that a filter may be used to select. the upper sideband v.and to suppress the lower sideband as well as any repeated components of the image band. By selecting the 4upper sideband (180.5 to 265 kilocycles) of the resultant modulation products and combining it with a carrier of 165 kilocycles in lav second modulator the output current of this modulator will contain an unmodulated carrier component of 15 kilocycles and two sidebands respectively comprising a lower sideband extending from `15.5 kilocycles to 100 kilocycles and an upper sideband extending from 345.5 kilocycles to 430 kilocycles, and the lower band may be selected by a filter.

At the receiving station, by supplying oscillations of, for example, 210 kilocycles to the rst demodulator, its output circuit vwill include Ademodulation products lying between 110 kilocycles and 310 kilocycles, which are so spaced in the frequency spectrum 4with respect to the incoming wave of 14.5 kilocycles to 100 kilocycles impressed upon the input lof the demodulator that a filter may be used to select these products and suppress the repeated wave, or to select one side- .band thereof and suppress the other sideband as well as an'y repeated components of the incom- In either case, the energy impressed upon the second demodulator occupies a'position in the frequency spectrum such that a filter in its output circuit will select the reproduced image current and suppress the repeated current components.

In the preceding explanation, relating to modulation and demodulation, the elect of harmonics produced by the modulators and demodulators has not been referred to, since they result in the production of currents which will be suppressed bythe lters and, in any event, are of negligible amplitudes.

Each subscribers receiving equipment includes a dial D, which is `connected over a simplex circuit of the line SL to control a suitable line relay LR, through which impulses for actuating the stepping magnet SM may be, relayed. Briefly described, the program` selecting means is controlled by actuating dial D to the digit identifying the program it is desired to receive.

As the'dial D is moved off-normal, its olf-normal contact 54 is closed, so that the midpoint 0i' the bridge 55 is grounded, and the circuit is completed, through the low-pass` filter 43 to the midpoint of the secondary winding of transformer 4|, conductor 56 to the midpoint of the primary winding of transformer, over the two .sides of the line SL in parallel to the midpointof the secondary winding 40, and thence through the .trol circuit forl relay 38 which operates to disconnect the primary winding of transformer 39 from the switching apparatus during the switching operation. This circuit extends from ground through the left hand front contact and armature 59 of slow release relay, winding of relay", battery 50 .to ground. This prevents the dialing impulses, when the dial is released, from being transmitted over the various switching contacts to other subscribers lines which may be connected to other program circuits wired to these' contacts.

As soon as the dial is released, it returns and sends interrupted impulse to the line relay LR, the slow release relay SR holds up during the interruptions, and each time the line relay falls oil, a circuit is completed over its back contact, the right hand front contact of the slow release relay SR to the stepping magnet SM, which steps the wipers 3l of the switch banks in a counter-clockwise direction until the wiper rests on the particular contact desired. Shortly after the last impulse is received the slow release relay is deenergized to open the circuit through-the stepping magnet and relay 38, whereby the subscribers line is connected to the desired program trunk.

If the subscriber desires to select a different program, he ilrst manipulates the dial D to dial 0. As soon'as the dial is olf-normal, the linerelay LR is energized to pull up the slow release relay SR and operate the relay 38. Upon releasing the dial, the pulses are transmitted to the line relay which operates the stepping magnet to advance the wipers to the next zero contact of each bank. This will in general require less than all of the ten pulses of the zero group. when this point is reached, ground is applied over the zero contact 5I of row i2, the inner left hand contact of slow release relay SR, winding of Y stepping magnet SM and battery to ground. This holds the stepping magnet locked up while the remaining impulses of the zero group are being transmitted. After the last impulse is received, the slow release relay SR releases and disconnects ground from the stepping magnet, which, upon release, advances the wiper 53 to the next or normal contact 5l'. The selecting mechanism diagrammatically illustrated and briefly described above, corresponds in designA and operation Ato that disclosed in United States Patent 1,685,835, issued October 2, 1928 to Bascom et al., to which patent reference may be made for a complete description of its construction and mode of operation. If desired, the dial and locking key of Fm. 2 of that patent may be substituted for the dial D for the purpose of controlling the program selector in-th'e manner therein described.

- Fig. 3=illustrates a simplified transmitting termina] apparatus which may be substituted for that shown in Fig. 1, to the left of section line A-A. In this arrangement, the sound and television pickup devices TP and TS of Fig. 1 are respectively connected to the sound amplifier 25 and a television amplifier G4. The audible sound currents are transmitted through the filter 29A to the amplifier 55. The television band is split into two portions by the low and high pass lters II and I2 and the lower portion is supplied to the modulator I3 in which it is combined* with a 15 kc. carrier current supplied by the source Il, the two sidebands and the unmodulated carrier being transmitted through lter I5 to the amplifier 55. Simultaneously. the upper portion of the image band transmitted by the filter I2 is combined in the modulator with oscillations of 180 kc. supplied by the source 55. One sideband of the mod- Y ulation products thus produced is selected by illter 2I and combined in modulator 22 with oscillations of 165 kc. supplied by the oscillator 51. Filter 24 serves to select one sideband, extending from 15.5 kc. to an upper limiting frequency,l which is also supplied to the amplifier 55. The amplified image modulated currents and sound currents are supplied over a trunk T to the ampliner 'I5 and program selector PS at the central omce as shown in Fig. 1.

Fig. 4 illustrates a second form of program transmitting apparatus which, with proper selection'of frequencies, may be substituted for that shown to the left.of section line A-A of Fig. 1. In this arrangement the sound and television pickup devices TP and TS are respectively connected to the sound amplifler 25 and television amplifier 5l. As in the preceding systems, the sound currents after being amplied by amplifier 25 are transmitted through filter 29 and the image band is split into lowerand upper portions by the low and high pass filters II and I2. 'Ihe lower portion is supplied to the modulator I2 in which it is combined with a carrier frequency supplied by source I4 to produce two sidebands and an unmodulated component which are transmitted through i'ilter I5.

The upper portion of the image band is applied through a transformer 58 connected in the common connection extending from the filaments of two space discharge devices 59 and III to their grids. The discharge devices 58 and 1l are connected in push-pull relation and are included in a balanced circuit. The grids of these devices are connected by a circuit including the secondary winding of a transformer 1I, the primary of which is supplied with carrier current by the source Il. The grids of the devices 69 and 18 are polarized from a source 'I2 Vto cause them to operate as a modulator as disclosed in United States Patent 1,343,307, issued June 15, 1920 to Carson. 'I'he upper portion of the image band, being applied'through a transformer 68 will be combined with the oscillations supplied through the transformer 1I `to produce in the winding 13, included in its output circuit, two sidebands and an unmodulated component of the frequency of the oscillations supplied by source Il. However, with the circuits of the two devices accurately balanced, no components of the portion of the image band applied through transformer 58 will flow through winding 13.

One sideband of the modulation products, induced in the winding 14, is selected by the filter 2l and is supplied, along with the sound currents and the modulation products transmitted by lter I5, to theA amplifier 55, which, as in Fig. 3,

is connected by,a trunk T to the ampliiier 15, connected by circuit 3| to program selector PS at the central oilice.

The transmitter of Fig. 4 constitutes a simplifled arrangement in which the image current components repeated by the discharge devices 59 and 18, constituting the modulator, are balanced out and hence are not transferred to the circuit including the lter 2l.

For a complete disclosure of the operation of Y the balanced modulator, briefly described above,

2,095,360 reference may be made to the above mentioned 'Carson patent.`

Modulators I3, I8 and 22 may be of the balancedtype and may have the image `and carrier currents applied thereto as shown in Fig. 4. InV place of the synchronizing system of Manderfeld, the transmitting scanning and receiving discs maybe held in synchronism as disclosed in United States Patent 1,860,936 ,of W. A. Marrison et al., issued May 31, 1932. Again, synchronous operation of the discs 3 and IDI may be secured by providing synchronous motors 3a and |02 'and supplying them with 60 cycle current from a commercial network, in accordance with present day practice in television broadcasting systems:

Figure 5 illustrates an alternative receiving apparatus which may be substituted for that shown in Fig. 2, and might well be associated with the transmitting arrangement shown in Fig. 4. The

arrangement shown in Fig. 5 is identical with that of Fig. 2 except that in the part of the receiving circuit lying between Athe high-pas'slfilter 41 and the television receiver, only a single step of demodulation is employed. In the arrange-` and Flg.v5 that in order to employ asingle step of modulation or demodulation in connection with a wide signal band, it is ydesirable that the frequency of the carriercurrent employed should not-be too low. If the upper limiting frequency v of the carrier sideband is more than three times its lower limiting frequency, it will be found that the unwanted sideband, resulting from the modulation process or to be demodulated, will overlap the' wanted sideband in such va manner that interference may result. For example, if a signal band of 10 cycles to 40 kc. is used to modulate a carrierl of l5 kc. the upper sideband will extend from kc. to 15.1 kc. and the lower sideband will extend from,14.9,kc. to zero and from zero to Hence both sidebands will include current components in the frequency range between 15.1 kc. and 25 kc. This diculty may be avoided by suitable selection of frequencies. Thus, in the arrangements of Figs. 4` and 5 if the upper limiting frequency of the single sideband representing the television image were selected as 100 kc., the lower frequency of this band should be as least 33.3 kc.

Fig. 6 illustrates a simplified arrangement of the transmitting terminal apparatus which may be substituted for that shown in Fig. 1 to the left of the section line AA. In this arrangement, which does not involve splitting the television band, one sideband of the image modulated wave is selected and transmitted to thel central oflice over a trunk which is shown as common to both television and sound currents.

The sound and television pickup devices TP and TS of Fig. 6 are connected respectively to the sound amplifier 25 and a television amplifier 64. The audible sound currents are transmitted through the lter 29 to the amp1ifer65, from which point they pass to the trunk T and` thence to the central office.` The television image band,

l above the audible sound frequencies.

after being amplified by amplifier 64,l is stepped up in frequency by modulatorll to a frequency range well above its original .position and of such frequency as to permit a filter to select one sideband. Afllter 8| selects the upper sideband and may also afi'ord partial transmissionof some components of the lower sideband. Theputput of this filter is passed on tomodulator 82"\`Where it is combined with a' carrier frequency to produce, among other components, an image modulated sideband lying in the frequency range slightly Modulator 82 is supplied with current of suitable carrier frequency from oscillator 81 through a harmonic producer 86 and tuned circuit 85. The carrier frequency for modulator 80 is supplied from the saine oscillator 81 through a harmonic producer 86 and tuned circuit 84. In the output of the modulator 82, the image modulated sideband, just above the audibe frequency range (with possibly certain partially suppressed components of the opposite sideband), is selectedby filter 83 and is combined with the audible frequency band and both sound and television signals are impressed upon amplifier 65. From. amplifier the sound and image modulated bands are transmitted over i a common trunk I1 to the central office Where they are amplified by means of amplifier 15 and supplied over wires 3l to the program selector PS. yFor purposes of illustration, a method of operation involving the use of certain frequencies is indicated in connection with Fig. 6, but it should be understood that the arrangement is not limited to these frequencies, nor to the use of an upper or a lower sideband precisely 'as described in each case.

In the arrangement of Fig. 6 the desired sideband is produced by two steps of modulation as a preferred arrangement, but with proper selection of frequencies one step of modulation may be` used to obtain the desiredsideband.

Fig. '1 illustrates still another arrangement of the transmitting terminal apparatus which may be substituted for that shown in Fig. 1` to the left of section line A--A. In this arrangement, `which also does not involve splitting of the television band, both sidebands of the modulated image band are transmitted to the central offlce vover a trunk which is common for both image modulated and sound currents. According to this arrangement, sound and television pickup devices TP and TS as in Fig. l are respectively connected to the sound amplifier 25 and television amplifier 64. As before, the audible sound currents are transmitted through the filter 29 to the amplifier 65. 'I'he television image band,

after being amplified by amplifier 64, is stepped up in frequency by modulator 80 to a frequency position well above its original position. The unmodulating component and both sidebands are then selected by filter 9i and passed to modulator 92 where they are combined with a carrier frequency which is of such value that one of the pairs of sidebands produced will be just above the audible frequency range. The modulator 92 is supplied by oscillator 91, which also is the source of carrier frequency for modulator 96, through the harmonic producer 96 and the tuned circuit 94. A lter 93 selects the lower pair of sidebands produced by the modulator 92, these sidebands being in the frequency range just abovefhe audible frequency range as determined by the choice of a proper frequency for oscillator 91.

bined with the voice band and passed through central oilice. At this point they are amplified by means of ampliiler 15 and supplied over wires 3l to the program selector PS. Again in this case certain frequencies have vbeen shown on the figure merely for purposes oi.'v illustration.

While in the arrangementillustrated in Fig. 7 the desired sidebands are produced by means of two steps of modulation, as the preferred arrangement, one step of modulation may be substitute'd ii' the frequencies are properly chosen.

Another form of the invention is illustrated in Fig. 8. In this arrangement the sound and television signals are transmitted from their source oi origin over separate pickup circuits to the program center, at which point they are separately amplified and passed over separate trunks to the central oillce. At the central omce they are individually amplied and transmitted through the necessary selecting equipment to the subscribers loops. Two loops are associated with each subscriber and the voice signals are transmitted over one loop while the television signals are transmitted over a second loop. At the subscribers premises these signals are separately amplified and reproduced in the form of audible sound and television images. A dial -at the subscribers station is operated to control the selection of the desired sound and television signals and to connect the trunks carrying them to the subscribers loops. y

In more detail, the audible program is picked up by the device TP, amplied through amplifier `25a. and transmitted over a pickup circuit 28a. to the program center at which it is amplified by amplifier 25. From this point it-is transmitted over trunk 28 to the central oillce where scribed.

The television signals, after `being originated by means of the mechanism TS, are amplified through amplifier Ila and transmitted to the program center over a trunk 26a' separate from circuit 26a. At the program center, after being amplified by amplifier lll, the image signals are transmitted over trunk 2i" to the central oillce and through` appropriate selecting apparatus, working in common with the audible signal selecting apparatus to the subscribers loop SL'. At the subscribers station the signals are supplied through amplifler 5l to the television receiver which may be used for directly viewing the image, a suitable receiver for projecting an image upon a screen may be substituted therefor. The television circuit is selected simultaneously with the voice circuit under the control of dial 5I, the switching apparatus for the television circuit being'operated simultaneously with that for the voice circuit by means of stepping magnet SM in' a manner already described. An additional relay 35a is included in series with relay 38 to remove the switching apparatus from the television circuit during the switching operation, and additional banks of contacts are provided to peramplifier 55 and over a common trunk T to the mit the simultaneous switching of sound and television circuits. Y

'I'he invention herein described is not confined to television images of stationary or moving objects. since by substituting a motion picture lm at the television source the arrangement is suitable for the transmission of moving pictures.

The television and voice signals may be made to modulate carrier frequencies located` in the frequency range above the audible range. Both voice and television modulated signals may then be superimposed upon a working telephone system without interfering in any way with the telephone conversations.

What is claimed is: v

l. The method of producing carrier currents modulated with an image` frequency band, having frequencies so low that when a carrier is modulated therewithl the frequency separation between the two sidebands is too small to permit effective separation of the sidebands by filters, to eifect substantially single sideband transmission, which comprises dividing the image band into lower and upper portions, modulating a carrier current with the lower portion of the image band to produce two sidebands and an Vunmodulated component of carrier frequency, effecting separate modulation with the upper portion of the image band to produce modulation products including two sidebands, the lower frequency component of the'lower sideband having the same frequency as the upper limiting frequencyof the upper sideband of the first modulation and with the sidebands of said separately produced modulation products so spaced that they may be selectively separated from each other, selecting from said separately produced modulation products said lower sideband, and combining said selected sideband with the modulation products of said first modulation to produce for transmission modulation products including modulated components and an unmodulated component which combineto faithfully reproduce said image current-band.

2.1The method of producing carrier currents modulated with an image frequency band, having frequencies so low that when a carrier is modulated therewith thev frequency separation between the two sidebands is too small to permit effective separation of the sidebands by-fllters, to eifect substantially single sideband transmission, which comprises dividing the image band into lower and upper portions, modulating a carrier current with the lower portion of the image band to produce two sidebands and an unmodulated component of carrier frequency, modulating a carrier of highv frequency, compared with the first mentioned carrier, with the upper portion of the image band to produce sidebands `which are readily separable by filters, selecting one of the sidebands of(said high frequency modulation, combining said selected sideband with a wave of such frequencyV as to produce sidebands, the frequency of the lowermost component of which is that of the upper limiting component of the modulation products of said rst modulation, selecting the lower of said last mentioned sidebands and combining it with the modulation products of. said first modulation to produce for transmission modulation products including modulated current components and an unmodulated current component which may be combined to faithfully reproduce said image current band. -3. An apparatus for producing a carrier curl acca-,seo

Y age band to produce a carrier modulated current -including two side bands, means for separately modulating said carrier with said upper portion lof the image band to produce sidebands spaced apart so that one may be selected while the other is suppressed, means for selecting one of said sidebands and combining it with the two .first mentioned sidebands to produce a modulated current of substantially the same band width as the image current ,andv including modulatedand unmodulated components which combine to faithfully reproduce said image current band, and means for supplying said combined.

current to a transmission circuit.

4. A combined television-sound system com- ,prising a program center including a plurality of simultaneously operable program producing devices each having means for producing image and audible frequency currents corresponding to related sound, means for producing an'image modulated wave including' an unmodulated carrier component,` having substantially the same frequency band width as the image current and occupying a diiierent'frequency range than the audible frequency current, a central oce remote from said program center, a plurality of pro- `gram trunks, equal in number to said program producing devices, connecting said program center and said central oflice, means for transmitting a program comprising said image modulated and associated audible currents lover each of said trunks to said central oice, switching means at said central office for connecting any one of said trunks to any subscribers line of the system, and each subscribers station including a program receiving equipment comprising means for separating said image modulated and audible frequency currents, means controlled by said modulated wave for producing an image vof a subject scanned at said program center, and means for reproducing the related sound, and, in addition, means for controlling the actuation of said switching means to connect the subscribers line at any time to a desired program trunk.

5. A combined television-sound system comprising a program center including a plurality of simultaneously operable program producing devices each having means for producing image and audible frequency currents corresponding to related sound, means for producing an image Vmodulated wave having substantially the same frequency band width as the image current and anv unmodulated carrier component combinable With the. modulated components to reproduce the image current and lying in a different frequency range thanl the audible frequency current, a central office remote fron'isaid program center, a plurality of program trunks, equal in number to saidprogram producing devices, connecting said program center and said central oiiice, means for transmitting a program comprising said image modulated and associated audible currents over each of said trunks to said central omce, switching means at said central oilce for connecting any one of said trunks to any 4subscribers line of the system, and each subscribers station comprising a program receiving equipment including means for separating said image' modulated and audible frequency currents, means controlled by the modulated and unmodulated components of said modulated wave to produce plying a pluralityof different programs thereto,

` circuits extending from said central oice to a plurality of subscribersstations respectively, switching vmeans located at said central oiiice for controlling the application of any of said programs to any of said subscribers circuits at any time, means at each subscribers station for controlling the actuation of said switching means, andl means at each subscribers station for reproducing a sound program and a television program.

7. In a system for distributing sound and television programs, a plurality of lines terminating at subscribers stations respectively, a plurality of program trunks, equal in number to the number of said programs, over which program transmission is effected, switching means'remote from the sources of said programs, for connecting any and means at each subscribers station for controllingthe actuation of said switching means to connect'his line to a program trunk carrying a desired combined television and sound program.

8. A system for distributing television programs which comprisesa plurality of sources of television signals, transmission circuits extending from saidsources Vto a central distributing sta-` tion, means for amplifying the television signals at said central distributing station, trunks, equal in number to the number of said programs, extending from said central distributing station to each of a plurality of intermediate distributing stations, means at said main distributing station `for simultaneously supplying said programs to the trunks respectively individual to each of said intermediate distributingstations, means at each' intermediate station for amplifying the television signals, circuitsextending from each intermediate station to a plurality of subscribers stations,

and means at each subscribers station for reproducing a television image.

9. A system for distributing television programs which comprises a plurality of sources of television signals, transmission circuits extending vfrom said sources to a main distributing station,

'of said lines to any of said trunks at'any time,

means Afor amplifying the television signals at said main distributing station, trunks, equal in number to the number of said programs, extending from said main distributing station to each of Aa plurality of intermediate distributing stations,

switched to select any one of said television programs.

10. A system for distributing television programs which comprises a plurality of sources of television signals, transmission circuits extending from said sources to a main distributing station, means for amplifying the television signals at said main distributing station, trunks, equal in number to the number of said programs, extending from said main distributing station to each of a plurality of intermediate distributing stations, means at said main distributing station for simultaneously supplying said programs to the trunks respectively individual to each of said intermediate distributing stations, means at each intermediate station for amplifying the television signals, circuits extending from each intermediate station to a plurality of subscribers stations, switching means located at each intermediate distributing station by which each subscribers circuit may be connected to a desired television program, means' vision programs which comprises sources of sound signals and of television signals, circuits extending from said sources to a main distributing station, means foramplifying the television signals and the sound signals at said main distributing station, trunks, equal in number to the number of said programs, extending from said main distributing station to each of a plurality of intermediate distributing stations, means at s aid main distributing station for simultaneously supplying said programs to the trunks Irespectively individual to each of said intermediate stations, means at each intermediate distributing station for amplifying the television signals and the sound signals, circuits extending from each intermediate distributing station to a plurality of subscribersv stations, 'and means at each subscribers station for reproducing a sound program and a television program.

12. A system for distributing sound and television programs which comprises sources of sound signals and of television signals, circuits extending from said sources to a main distributing station, means for amplifying the television signals and the sound signals at said main distributing station, trunks, equal in number to the number of said programs, extending from said main distributing station to each of a plurality of intermediate distributing stations, means at said main distributing station for simultaneously supplying said programs to the trunks respectively individual to each of said intermediate stations, means at each intermediatedistributing station for amplifying the television' signals and the sound signals, circuits extending from each intermediate distributing station to a plurality of subscribers Stations, means at each subscribers station for reproducing a sound program and a television program, and means under the control of the subscriber for selecting a. television and associated Sound program.

13. A system for distributing sound and television programs which comprises sources of sound signals and of television signals, circuits extending from the respective sources to a main distributing station, means at said main distributing station rfor separately amplifying the sound signals and the television signals, means at said main distributing station for shifting the telei vision signals to a frequency ra e above that of the sound signals, trunks, equ l in number to the number of programs, extending from said main distributing station to each of a plurality of intermediate distributing stations, means at said main distributing station for simultaneously supplying said programs to the trunks respectively individual to each of said intermediate stations, means at each intermediate distributing stationfor separately amplifying the television signals and the sound signals, means at each intermediate distributing station for associating each set of television signals with a corresponding set of sound signals in a common circuit, circuits extending from each intermediate distributing station to a plurality of subscribers stations, and means at each subscribers station for reproducing a sound program and a television program.

14. A system for distributing sound and television programs which comprises sources of sound' signals and of television signals, circuits extending from the respective sources to a main distributing station, means at said main distributing station for separately amplifying the sound signals and the television signals, means at said main distributing station for shifting the television signals to a frequency range above that of the sound signals, trunks, equal in number to main distributing station to each of a plurality of intermediate distributing stations, means at said main distributing station for simultaneously supplying said programs to the trunks respectively individual to each of said intermediate stations, means at each intermediate distributing station for separately amplifying the television signals and the sound signals, means at each intermediate distributing station for associating each set of television signals with a set of sound signals in a common circuit, circuits extending from each intermediate distributing station to a plurality of Asubscribers stations, means at each subscribers station for reproducing a sound program and a television program, and switching means under the control of the subscriber for automatically selecting a particular television and associated sound program for his station.

l5.` The method of transmitting a band of television signals extending from substantially zero frequency to Some upper limiting frequency which consists in dividing the television band into two parts, transmitting the lower of said parts as a double sideband of a carrier frequency, and transmitting the upper of said parts as a single sideband of the same carrier frequency.

16. The method of transmitting a band of television signals extending from substantially zero frequency to some upper limiting frequency which consists in dividing the television band into two parts, passing the lower of said parts through a single stage of modulation and transmitting this part as a double sideband of a carrier frequency, passing the upper of said parts through two stages of modulation and transmitting this part as a single sideband of the same carrier= a,oes,aeo l "3 1 utingstation to each of a plurality of distribut for transmitting them from the central distributing point to the subscribers stations, and means j for transmitting the sound 'currents -from said sound sources to the subscribers' stations without of said programs may be transmitted' from said central distributing point to any of said subscribers stations.

19. A system for distributing associated sound and television programs which comprises sources of sound signals and sources of television signals, circuits extending from each of said sources of sound signals to a main distributing station, separate circuits extending from each of said sources of television signals to said main distributing station, means for amplifyingseparately the sound signals and the television signals at said main distributing station, separate trunks for each of the sound and each of the television signals extending from said main distributing station to each of a plurality of intermediate distributing stations, means at each intermediate distributing station for separatelyampliiyingthe sound signals and the television signals, a separate circuit for the sound signals and a separate circuit for the television signals extending "from each intermediate distributing station to each of a plurality of subscribers' stations, means at each subscribers station for reproducing a sound program and a television program, and means under the control ofthe subscriberfor, at all times, automatically and simultaneously selecting any one of said television Aand associated sound programs.

20. A system for distributing television programs which comprises a `plurality of sources of television signals, transmission circuits extending from said sources to a main distributing station, means for amplifying the television signals at said main distributing station, trunks, equal in numto the number of said programs, extending from said main distributing station to each of a plurality of intermediate distributing stations, means at said main distributing station for simultaneously v supplying each program to associated trunks respectively individual tosaid intermediate stations,` means at each intermediate station f or amplifying the television signals, circuits extending from each intermediate vstation to a.v plurality of' subscribers stations, switching means located at each intermediate distributing station by which each subscribers circuit may be connected toa desired television program, means at 'each subscribers station whereby said switching means may be actuated at any time tog effect the connection ofhis Vcircuit to a desired television program. Y n

2l. A system for distributing sound and television programs which comprises sources of sound .signals and of television signals, circuits extending from said sources to a main-distributing station, means for amplifying the television signals and the sound signals at said main distributing station, trunks, equal in number to the number of f said programs, extending from said main distribing stations, means -at said main distributing station for simultaneously supplying each program to associated 'trunks connected to the respective intermediate distributing stations, means at each intermediate distributing station for amplifying the televisionsignals and the sound signals, circuits extending from each intermediate distributing station to a plurality of subscribers stations, means at each subscribers station for reproducing a sound program and a television prbgram, and 'means under the control of the subscriber for selecting at any time one of said television and associated sound programs.

22. A system for distributing sound and televisionprograms which comprises sources of sound signals and of television signals, circuits extending from said sources to a main distributing station, means at said main distributing station for separately amplifying the sound signals and the television signals, means at said main distributing station for shifting the television signals toa frequency range above that of the sound signals,

, tributing stations, means at saidmain transmission stations for simultaneously supplying the respective programs to a plurality of trunks each extending to one of said intermediate distributing` stations, means at each intermediate distributing station for separately amplifying vthe television signals and the sound signals, means at each intermediate distributing station -for associating each set of television signals lwith a set of sound signals in a common circuit, circuitsk extending from each intermediate distributing station to a plurality of subscribers stations; means at each subscribers station for reproducing a sound program and atelevision program, and switching means under the control of the subscriber for automatically selecting a particular television program and associated sound -forhis station at any time. v

23. A system for distributing associated sound and television programs which comprises sources of sound signals and sources of television signals,

circuits extending `from said sources of sound' signals'to a main distributing station, separate circuits extending from said sources of telenals and the television signals at said main distributing station, separate trunks for the sound and television signals extending from said main distributing station to each of a plurality of intermediate distributing stations, means at the main distributing station for simultaneously supplying the respective television signals to a plurality of.trunks respectively extending to said intermediate distributing stations and the respective sound signals to a plurality oit-trunks respectively extending to said intermediate distributing stations, means -at each intermediate distributing station for separately amplifying the sound signals and the television signals, separate circuits, for the sound signals and the telef vision signals extending lfrom each intermediate distributing station to a plurality of subscribers stations, means at each subscribers trol of the subscriber for automatically and simultaneously vselecting a particular televisionV and associated sound program at any time.

. 24. Means for utilizing an image current to control the production of an image modulated carrier wave extending over a continuous frequency band comprising means for splitting the image current into a plurality of portions, means controlled by one of said portions for producing a group of modulation products extending from a lower limiting frequency to an upper limiting frequency, means controlled by another portion for producing a group of modulation products extending from a lower limiting frequency, which is the same as the upper limiting frequency. of said rst group of modulation products, to an upper limiting frequency, and means for combining said first and second groups of modulation products. V

25. Means for utilizing current representing a single communication and including' current variations extending over a wide frequency range to control the production of a signal modulated carrier wave extending continuously over a section of the frequency spectrum comprising means for splitting the signal band into a plurality of portions, means Icontrolled by one of said portions for producing a group of modulation products extending from a lower limiting frequency modulation products extending from a'lower limiting frequency, which is the same as the upper limiting frequency of -said first modulation products, to an upper limiting frequency, and means for combining said groups of modulation products.

26. Means for utilizing an image current to control the production of an image modulated carrier wave extending over a continuous portion of the frequency spectrum which is substantially the same as the frequency range of the image current comprising means for splitting the image current into a plurality of portions, the frequency range of one of said portions being small, means controlled by said smaller portion of the image current for producing va group of modulation products including two side-bands, the upper limiting frequency of which has a predetermined value, means controlled by another portion of the image current for producing a group of modulation products comprising a single side-band and having a lower limiting frequency of the same value as the upper predetermined frequency of the ilrst group of modulation products, and means for combining said first and second groups of modulation products.

. ESTILL I. GREEN. 

